Jason Chen

Milk. It's great on cereal, with cookies, and in your mouth when you've eaten too many peppers, but horrible when you pour some and chunks fall into your glass. It's easy tell when milk is obviously expired—when it looks more like yogurt than liquid—but what about before that? How do you know if you're throwing away milk too early—or too late?

You may think you can tell if a milk is still good by the sniff test, or by a quick taste test, but unless your superpower is a heightened sense of smell, the smell test isn't all that accurate until quite a bit of time after milk has gone bad. All milk cartons have a sell-by or expiration date stamped on them, and because we all hate throwing away food, we treat these as more of a suggestion than a rule. But is that right?

We asked some food safety experts whether or not it's safe to drink milk after the printed expiration date. And if so, for how long? Here's what they said.

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The sell-by date and the expiration date

Peter Bonet of the Lincoln Culinary Institute says the sell-by date is the date in which your supermarket needs to stop selling that particular carton of milk. The expiration date is the date you should, technically, throw that milk away, provided you stored the milk correctly. So it's actually a good guideline to go with, despite what your cheap parents may have had you believe.

Drew Harris, chair of the New Jersey Public Health Institute, says that these dates aren't actually standardized like you think they would be.

Fresh milk is regulated by each state and not the feds. So, there is no one standard for calculating "sell-by" dates. Some states ban the sale of raw milk completely, which is a much more likely to be contaminated. There's a reason Dr. Pasteur invented his process for disinfecting milk.

He points to a paper from the Cornell University Milk Quality Improvement Program (it's a thing) that says that shelf-life is determined by refrigerating milk at marginal refrigeration temperatures—about 45º F—seeing how much bacteria is in the milk after X amount of days. Bonet recommends at least setting your refrigerator to 38º F or below.

The paper says that one of the measures of drinkability is the bacteria Standard Plate Count (SPC). Freshly pasteurized milk has less than 500/ml. If you reach 1000/ml, that implies "a potential contamination problem either in the raw milk supply (detected by Laboratory Pasteurization Count) or within the processing equipment." Then, you have the regulatory limit of 20,000 SPC/ml, for "as long as the milk is offered for sale."

If you're going to refrigerate your milk at 45 degrees (which you shouldn't), your milk will become undrinkable within 6 days. That's when SPC reaches over a million/ml, which is something you can actually taste.

They also have a whole lot of information on different types of bacteria, some of which only cause shelf-life problems later on. The take-away is that even under the best pasteurization processes and storage situations, a store's sell by dates should not be beyond 21 days after the milk left the cow. Even after 14 days, you're going to run into the late-stage bacteria that survive the pasteurization process (but only cause problems after the two weeks).

With this information, you can do a rough calculation. If you can store your milk for about five days after the sell-by date, subtract the current date from the sell-by date, add five, and you have the amount of days this milk will be good for. For example, if I buy milk today that has a sell-by date of Aug 27, it will be good for 15 days, or a little over two weeks. Or, if there's an expiration date on the carton itself, just follow that.

Does it matter what type of milk you buy and how you store it?

Drew Harris, chair of the New Jersey Public Health Institute says that there are actually quite a lot of variables to consider:

Pasteurized, raw, ultra-pasteurized, fat content, amount of time out of the fridge, door storage versus back of the shelf, open/partially filled container, etc. I assume the standards are written to take into consideration some of these conditions, but I don't know how much time they allow you to consume the milk at home.

But, of course, you should always store your milk in the refrigerator and not let it sit out on the counter for an hour or two before you put it back in, to prolong its life expectancy.

If you're interested in getting as much life out of your milk as possible, say if you really hate going grocery shopping, Bonet says you can buy ultra pasteurized milk. "The milk is pasteurized longer and hotter to kill more of the bacteria which causes the milk to spoil."

Does it matter what type of person is drinking it?

Harris says you should also pay attention to what type of person is drinking the milk, meaning that if you fall into the following categories, it's better to throw out milk a little early.

Individuals also vary in their susceptibility to bacterial infections. The very young, old and immunocompromised need to be very careful. Also, some bacteria don't produce noticeable smells so don't assume it's safe if it passes the sniff test.

He also says you should pay attention to who else drinks your milk.

I would also assume that the backwash from your roommate's surreptitious midnight gulp is likely to introduce bacteria into the milk, hastening spoilage. Of course, we never do that.

Acknowledgements and what I've learned

A properly refrigerated thanks to our food experts, who've shown me that those sell-by dates and expiration dates aren't just there to get you to throw away your milk early so they can sell more.

Drew Harris is the chair of the NJ Public Health institute, and has previously written about public health topics here. You can find his personal site here as well.

Peter Bonet is an instructor at the LIncoln Culinary Institute in West Palm Beach Florida, and if you want to a proper food education, his site is here.

Do it right is a section where we explore common activities that we all think we're doing correctly, but might not be. And if you know someone who insists that they're doing something right, feel free to pass this along to show them what the experts say. Do you want to know if you're doing something right? Email us at doitright@lifehacker.com and we'll find out